Unit-VI Instrumental analysis
Unit-VILIQUID CRYSTALS AND THEIR APPLICATIONSIntroduction: The study of liquid crystals began with an observation made by Austrian botanist, Freindrich Reinitzer in 1888. He observed that solid cholesteryl benzoate on heating becomes a turbid liquid at 1450C which on further heating turns into a clear, transparent liquid at 1780C. The above changes are reversed on cooling. The changes are generally represented as
Cholesteryl benzoate is said to exit a liquid crystal between 1450C and 1780C. The first temperature at which solid changes into turbid liquid is known as transition point and second temperature at which turbid liquid changes in to clear liquid is known as melting point. Thus, liquid crystal is a distinct phase observed between crystalline solid state and isotropic liquid state. Definition: Liquid crystals may be described as a distinct state of matter in which the degrees of molecular ordering lie intermediate between the ordered crystalline state and the completely disordered isotropic liquid state.The liquid crystal state is also referred to as mesophase. The compounds which exhibit mesophase are also called mesogens. Liquid crystals exhibit optical anisotropy, i.e., they possess different optical properties when light is incident in different directions. Liquids, however, exhibit optical isotropy i.e., they exhibit same optical property irrespective of the direction of incident light.Positional and orientational order: Most liquid crystals are composed of organic molecules. In solid state, the molecules are highly ordered. Each molecule occupies a definite position in a more or less rigid arrangement and is immobile. In solid state not only do the molecules occupy specific positions but also tend to orient in a preferred direction which is already existing. In liquid state, however, the molecules neither occupy specific positions nor remain oriented in a particular manner. The molecules are somewhat free to move at random and collide with one another abruptly changing their positions. Intermediate between the solid and the liquid crystal phase, wherein the molecules free to move but are oriented in a particular manner.
Fig.1 Representation of solid, liquid crystal and liquid states (molecules are represented as thin lines)
Thus solid phases possess positional order and orientational order. Liquid phases possess neither positional nor orientational order while in liquid crystal phase some orientational order is retained through there is a loss of positional order as shown in Fig.1
Director: In a liquid crystal, the molecules possess orientational prder i.e., the molecules tend to remain oriented in a particular direction. The direction of preferred orientation in a liquid crystal is called director and may be imagined to be directed towards the top or bottom of the page. Since the molecules are in constant motion, in liquid crystal phase they spend more time pointing along the director than along any other direction. The extent of orientational order can be described by taking the average. A snapshot of the liquid crystal at any instant of time will give the angle made by each molecule with the director at any instant of time. If we consider a representative group of molecules and measure the angles using the snapshot, the average angle gives the measure of orientational order. An average of 00 indicates perfect orientation and can be expected in solids. An average of greater than 450 indicates no orientational order a found in liquids. However, in liquid crystals a smaller average angle with the director is observed which indicates some orientational order (Fig.2).
Fig.2 A snapshot showing the orientation of the molecules in the liquid crystal phase as compared to its solid phase. In the liquid crystal phase, the molecules orient in a preferred direction along the director with an average angle of 0. Change in the positional order could be seen.
Classification of liquid crystals: Liquid crystals are classified into two main categories, namely,
1. Thermotropic liquid crystals
2. Lyotropic liquid crystals.
1. Thermotropic liquid crystals: The class of compounds that exhibit liquid crystalline behaviour on variation of temperature alone are referred to as thermotropic liquid crystals. The temperature range at which some liquid crystal are stable are given below:SolidLiquid crystalLiquid
2. Lyotropic liquid crystals: Some compounds transform to a liquid crystal phase when mixed with a solvent. They have a lyophilc and a lyobhobic end that is they are amphiphilic compounds. They are usually obtained by mixing the compound in a solvent and increasing the concentration of compound till liquid crystal phase is observed. Such liquid crystals are called lyotropic liquid crystals. The formation of lyotropic mesophases is dependent on the concentration of either the component or the solvent. Variation of temperature also affects the formation of these mesogens. Examples: (i) soap (soap-water mixture) molecules
(ii) Phospholipids (biologically important molecules where each cell membrane owes its structure to the liquid crystalline nature of the phospholoipid-water mixture).Lyotropic mesogens are typically obtained from amphilic compounds comprising of both lyophilic (solvent attracting) and lyobhobic (solvent repelling) parts in the same molecule. In the presence of solvent the lyophobic ends come together while the lyophilic ends directed towards water forming micelles. The formation of micelles takes place only beyond a particular concentration of the solution called critical micelle concentration (CMC). When the concentration of the solution is increased (beyond cmc) the micelles increase in size and eventually coalesce to form liquid crystalline phase.Molecular ordering in liquid crystals (Types of mesophases):
(1) Nematic phase: Nematic (Greek nematos = thread like) liquid crystals are formed by compounds that are optically inactive. The molecules have elongated shape and are approximately parallel to one another (Fig.3). Nematic phase is characterized by the total loss of positional order and a near normal flow behaviour similar to its liquid phase.
Fig.3 Nematic liquid crystal
Examples: (i) p-azoxyanisole (116-1350)
(ii) p-azoxyphenetole (137-1670C)
(iii) Anisaldine (165-1800C)
(iv) p-Methoxycinnamic acid (170-1860C)
(2) Chiral (Twisted) Nematic Phase (Fig.4): Chiral nematic liquid crystals, also referred to as cholesteric liquid crystals or twisted nematic liquid crystals (TNLC), re formed from optically active compounds having chiral centres. Unlike in a nematic phase where all the molecules approximately parallel to one another, in chiral nematic phase, the molecules arrange themselves in such a way that they form a helical structure. In this mesophase, the director is therefore not fixed in space as in space as in a nematic phase, but rotates throught the sample forming a helical pattern as it changes its direction just like the motion of a nut on a screw. The distance traveled by the director as it completes one full turn is called the pitch of the liquid crystal. In other words, the pitch length is the distance traveled by the director when it gets turned by 3600C. The twisted pattern repeats itself throught the liquid crystal phase.
The most striking feature of choleteric mesophase is its strong optical activity and selective light reflection, which are attributed to the twisted structure. The twist present in chiral nematic liquid crystal imparts spectacular optical properties which are made used of as thermochromic materials. The pitch is also temperature dependent and hence cholesterics are used in thermography.Examples: (1) Cholesteryl benzoate (2) Cholesteryl myristate and (3) Cholesteryl formate etc.
Fig.4(a) Illustration of the twisted structure in a chiral nematic phase of a liquid crystal as shown by the change in the direction of the director. Arrows show the orientation of the molecules. Pitch is shown as the distance traveled for the director to come back to its original direction. (b) A nut moves a certain distance when it makes a full turn (one pitch) and comes back to its original direction is as shown by the arrows.
(3) Smectic mesophase: Substances that form smectic phases are soap-like (in Greek, smectos means soap). In fact, the soft substance that is left at the bottom of a soap dish is a kind of smectic liquid crystal phase. In smectic mesophase, there is a small amount of orientational order and also a small amount of positional order. The molecules tend to point along the director and arrange themselves in layers. A snapshot would reveal that more number of molecules position in regularly spaced planes and a few molecules lie between the planes (Fig.5). That is, any one molecule would spend more time in these planes than between the planes. Based on the orientation of the director there are many types of smectic phases. If the director is perpendicular to the planes it is called smectic A (Fig. 5a) and smectic C if the director makes an angle other than 900 (Fig.5b). In smectic B phase, the director is perpendicular to the plane with the molecules arranging themselves into a network of hexagons within the layer.
Fig.5 Schematic representation of smectic mesophase where molecules lie on regularly spaced planes (in the form of layers). (a) Smectic A where molecules is perpendicular to the layer planes. (b) Smectic C where the molecules are tilted with respect to the layer planes.Examples: 4-n-Butyloxybenzylideneamino propiophenone (smecti A)
Terephthalylidene-bis-4-n-butylaniline (smectic B)
4,41-di-n-Heptyloxyazoxybenzene (Smectic C)
Discotic or Columnar Liquid crystalline Phase: Liquid crystals formed by molecules which have disk-like or plate-like structure are referred to as discotic or columnar liquid crystals.The simplest discotic phase is also called discotic nematic phase because there is orientational order but no positional order (Fig.6a). There is random motion of the molecules, but on an average, the axis perpendicular to the plane of each molecule tends to orient along the director.In the discotic or columnar phase, in addition to the orientational order present in the nematic discotic phase, most of the molecules tend to position themselves in columns (Fig.6b). The columns are arranged in a hexagonal lattice resembling a set of coins stacked as shown below (note: the coins in a stack have a great deal of positional order i.e., the coins are equidistant whereas the molecules in a columnar phase are stacked in random fashion).
Fig.6 Schematic representation of disc like molecules arranged in (a) Discotic nematic (b) discotic columnar liquid crystal phases
Liquid crystalline behaviour in homologous series: A series of compounds of the same type in which all the members have the same functional group and molecular formulae of adjacent members differ by CH2 is called a homologous series. The thermal stability of a liquid crystal compound may be altered by altering the molecular structure e.g., by increasing its chain length as in homologues series. PAA (p-azoxyanisole) series: The liquid crystal phase of p-azoxyanisole (PAA) is stable between 1180C and 1350C. PAA has more than 12 homologues which are formed when CH2 groups are added to its side chain as shown below.
A plot of transition temperatures against number of carbon atoms for PAA is as shown in Fig. 7(a)
(a) (b)Fig.7 Graphical representation of different phases and transition temperatures of the homologous series of (a) p-azoxyanisole (PAA) and (b) p-methoxybenzylidine-p-n-butylaniline (MBBA)
It can be seen from Fig.7(a) that, in general, the transition temperature of the liquid crystal, decreases with the increase in the number of carbon atoms in the side chain. The molecules with even number of carbon atoms generally have higher transition temperature than those having odd number of carbon atoms. When the alkyl group (side chain) contains 1 to 6 carbon atoms, liquid crystals show a nematic phase and when the number of carbon atoms is greater than 6, smectic phase is exhibited. Homologues containing 7 and 8 carbon atoms, however, show a transition from solid to smectic to nematic before melting to a liquid. Molecules with longer alkyl chains exhibit smectic phase. In PAA series, the alkyl groups are linked to benzene ring through an oxygen atom. In addition, the transition temperature of the PAA series make them unsuitable for display applications.p-Methoxybenzylidene-p-n-butylaniline (MBBA) series: The structure of which is given below
It may be noted that there is direct linking of alkyl chain to the benzene ring on one side (C4H9) where as the other alkyl (CH3) is linked through oxygen to the benzene ring. The molecules with odd number of carbon atoms generally have higher transition temperature than those having even number of carbon atoms. The transition temperature (21-700C) and various phases of first five homologues of the MBBA obtained by changing the length of the chain that does not contain oxygen (i.e., C4H9) is shown in Fig.7(b).The members of MBBA series do not exhibit smectic phase. All the members show transition from solid to nematic to isotropic liquid state.The transition temperatures of the compounds can thus be altered by changing the length of the flexible side chain at terminal position. Biphenyl and terphenyl systems carrying highly polarizable groups such as nitro and cyano show lower transition temperatures. Such molecules with low transition temperatures have extensive applications in liquid crystal display.
Electro-optic effect of liquid crystals: Nematic liquid crystals have rod like molecular structure and align themselves spontaneously along the director. Nematic materials have two dielectric constants- one in the direction parallel to the director and the other perpendicular to the director. Dielectric anisotropy () is defined as the difference between the dielectric constants parallel and perpendicular to the director. Similarly, the optical anisotropy (n) is defined as the refractive index parallel to the director minus the refractive index perpendicular to the director. These two properties are important for the electro-optic effects in liquid crystals.Effect of electric field: The director in a liquid crystal is free to point in any direction. But when a film of liquid crystal is placed between two plates of certain materials, director is forced to point along a particular direction when an electric field is applied. For example, when a film of liquid is placed between two specially treated glass s...